Today, about 10 million of people worldwide suffer from type I diabetes, which is also referred to as insulin-dependent diabetes mellitus. However, the number of affected people is estimated to increase dramatically and may affect as many as 25 million by 2010. Presently, research is conducted for trying to achieve permanent normoglycemia in patients with type I diabetes by introducing insulin-producing β-cells. The two main procedures have been transplantation of either vascularized pancreatic grafts or isolated islets of Langerhans. Although some success has been obtained with vascularized grafts (whole pancreas) problems still remain mainly due to the surgical risk and the post-operative complications. In addition, there is also a problem with shortage of suitable pancreatic graft donors. By contrast, transplantation of isolated pancreatic islets is conventionally performed by injecting the islets transhepatically into the portal vein, whereby the islets embolize in the portal tree of the liver.
A novel protocol for islet allotransplantation that was recently introduced by Shapiro and coworkers [1] will undoubtedly be beneficial to a number of patients with type I diabetes. However, even with this new approach, it has turned out that transplantation of islets from a single donor pancreas is not sufficient to produce normoglycemia in a patient [2]. As a result, the supply of human islets is expected to become a limiting factor in the treatment. Alternative sources of insulin-producing cells will then have to be found. One option is to use islets prepared from animal tissue, with islets from pigs being the chief candidate.
One of the main obstacles to be resolved before islet xenotransplantation becomes possible is the injurious inflammatory reaction that porcine islets elicit when exposed to fresh human blood in vitro and in vivo [3]. Also, human islets induce an injurious inflammatory reaction when exposed to ABO-matched blood of the patient at the time of intraportal transplantation [4]. The inflammatory reaction is characterized by a rapid consumption and activation of platelets, which adhere to the islet surface promoting activation of both the coagulation and complement cascades. In addition, the islets become embedded in clots and infiltrated by CD11b+ leukocytes, which all together results in a destruction of the morphology of the cells and loss of normoglycemia of the patients [3-4]. Furthermore, the inflammation may accelerate the succeeding cell-mediated specific immune response in a later phase [5-8]. Hence, inhibition of Instant Blood-Mediated Inflammatory Reaction (IBMIR), as the injurious inflammatory reaction is called appears to be critical to the success of islet allotransplantation and xenotransplantation.
Two recent studies by Buheler et al. [5] and Cantarovich et al. [6] have demonstrated that adult porcine islets are immediately destroyed when transplanted intraportally into the liver of non-human primates even under conditions of extensive conventional prior art immunosuppression. In these studies, the authors concluded that a powerful innate immune response, IBMIR, which is not affected by immunosuppressive drugs, is involved in the destruction of the xenogeneic islets.
Fiorante et al. have studied the use of dextran sulfate in preventing hyperacute rejection (HAR) of vascularized discordant xenografts [9]. Pig lungs perfused with citrate-anticoagulated human blood experienced HAR after 30 min in the xenotransplantation model. However, addition of dextran sulfate at 2 mg/ml prolonged lung survival to about 200 min. HAR of vascularized whole organs is mediated through the action of antibodies in the human blood, which identify and bind to exposed antigens on the endothelial cells of the blood vessels of the transplanted organs. This antibody-mediated HAR reaction is enhanced by components of the complement system [8, 10, 11]. Since dextran sulfate is also known to inhibit complement activation [9, 12], the prolonged lung survival when using dextran sulfate in the used xenotransplantation model is believed to derive from this anti-complement effect of dextran sulfate.
Nakano and coworkers have transplanted isolated syngeneic islets into livers of STZ-induced diabetic mice in order to investigate the roll of hepatocyte growth factor (HGF) in amelioration of hyperglycemia [13]. Dextran sulfate is known to enhance the effect of HGF and consequently HGF was administered intraperitoneally in the recipient mice in conjunction with dextran sulfate. Such administration produced normoglycemia in all mice under investigation. Also administration of dextran sulfate alone showed some beneficial effect in a few mice, but not when the renal subcapsular space was the site of islet transplantation. Additional anti-HGF antibody treatment to the dextran sulfate administered mice totally abolished the beneficial effect of dextran sulfate, indicating that the effect of dextran sulfate in this model of allogeneic islet transplantation in mice is mediated via endogenous HGF.
Thomas et al. [14] have demonstrated that soluble dextran derivates inhibit complement activation and complement mediated damage in vitro. Porcine aortic endothelial cells incubated in human serum resulted in complement consumption and deposition of activated fragments C3, C5 and of the membrane attack complex C5b-9 on the endothelial cells. Addition of 25 mg/ml of CMDB25 dextran sulfate inhibited complement activation and cytolytic complex deposition on the cells. Native dextran had no such an effect.